THE USE OF COMPUTER SIMULATION TO FIND EFFLUENT TREATMENT AND RECIRCULATION SOLUTIONS FOR SAPPI PULP AND PAPER MILLS

Authors

Maryna Mansfield and Volkmar Böhmer

Company and address

Sappi Technology Centre, Environmental Department, PO Box 3252, Springs, 1560, South Africa

emails

maryna.mansfield@za.sappi.com and volkmar.bohmer@za.sappi.com

Keywords

simulation, models, mass balance, pulp and paper mills, closed systems, zero discharge

ABSTRACT

Sappi mills in South Africa face several challenges regarding water and effluent handling. Some of these challenges are periodical droughts, increasing potable water prices and stricter effluent discharge standards. Because of these challenges, effective effluent treatment and recycling within a mill is becoming increasingly important. By using computer simulation techniques to test various treatment options, the task of finding the best solution is made much simpler. Many possible solutions can be simulated and compared, thus increasing the confidence in the ultimate solution. This paper describes how computer simulations can be used to find effluent management solutions and uses two case studies to illustrate the success of the technique.

1. INTRODUCTION

Pulp and paper mills in South Africa are facing stricter wastewater discharge regulations than in past years. Internationally, the environmental pressures placed on mills are expected to increase in years to come, as mills will be expected to discharge effluent of similar quality to the intake water1. As new regulations are enforced, mills that are not pro-active in their approach to effluent treatment, thus only striving for minimum compliance, are often left with effluent treatment equipment that is redundant or inadequate to meet new standards1. Being pro-active in effluent management involves the closure of water loops in the mill through recycling, thus reducing the mill's impact on the environment, often before it becomes mandatory for the mill to comply with these standards.

Besides legal requirements, there are several other factors that motivate partial or full closing of mill water systems2. They are:

  • Contaminant load reduction in the effluent discharged, which will improve environmental performance.
  • Unreliable availability of fresh water. In South Africa, this is a common occurrence, due to periodical droughts.
  • Potential cost savings through lower water abstraction and effluent discharge rates, if applicable.
  • Enhancement of the public profile of the mill.
  • Achieving an effluent-free mill means that there will be no more pressure for regulatory compliance or future costs for changing the treatment regime.

It is therefore important for mills to shift their effluent management focus towards achieving an effluent free mill, where there is only minimal impact on the environment. Although it might not be viable to implement such a system immediately, due to a lack of the motivation factors listed above, it is important to ensure that the mill is on the 'right track' with any effluent management strategy that is implemented. This ensures that all subsequent effluent treatment and recycling steps that are implemented by the mill will bring the mill closer to the ultimate effluent solution – the effluent free mill.

There are numerous effluent recycling and treatment methods that can be implemented to reduce the environmental impact of a mill2, 3, 4, 5. Although these methods, as well as all the advantages and disadvantages of each, are well known, each mill is unique and therefore requires a unique and practical effluent management solution.

Therefore, finding the solution that is not only technically but also economically viable is of great importance, and should be investigated thoroughly. All possible options have to be taken into consideration, which makes this task a daunting one. The use of computer simulations to predict the effect of each treatment or recycling option, or combination of options, greatly improves the confidence in the ultimate solution.

2. EVALUATION OF MILL EFFLUENT TREATMENT OPTIONS USING COMPUTER SIMULATION

Simulation software
There are several commercial process simulation software packages available, each having a specific target market.

WinGEMS (General Energy and Material balance System) is a modular program designed to perform mass and energy balance calculations. Calculations are grouped together in modules called blocks. The program has a wide selection of blocks that perform process calculations specifically for the simulation of pulp and paper systems.

WinGEMS uses a modular concept that makes it possible to simulate many different processes using a finite number of calculation blocks. A WinGEMS simulation project is created by diagramming a process using GEMS blocks and streams. The program then calculates the blocks iteratively, to converge on a solution6.

Computer simulation methodology
The process that is used for finding a zero effluent solution for a mill is shown in Figure 2.1. The steps that involve the use of a computer simulation are highlighted. The various inputs required and outputs produced by the computer simulation are also shown.

Figure 2.1

Figure 2.1. The stepwise process to finding an effluent management solution for a mill using computer simulation

The first and most important step in using a computer simulation to find an effluent management solution is to build a representative computer model of the mill. This model must be a mathematical representation of the mill unit operations, and has to take into account all the interactions between these units. During this stage, data is gathered from various sources, such as mill process diagrams, historical laboratory data, flow data and manual measurements, and transformed into a computer simulation that represents the mill operations, including all effluents generated by these operations.

Before starting the simulation, it is important to decide which key components will be incorporated into the model. Only the most important components should be included for simulation, as each additional component adds complexity to the simulation process and the resulting mill model. Each mill has different key components, depending on the process itself, as well as the environmental and process constraints for the mill.

Once the baseline model has been completed and verified, it can be used to simulate various treatment options.

The first step towards reducing the mill's environmental impact is to investigate possible in -mill water and effluent reduction measures. These may include internal process control issues, such as consistency control, or internal process water re-use, such as vacuum pump sealing water recycling. Using the baseline model as a starting point, the effluent quality resulting from implementing the in-mill recycling measures can be projected.

After obtaining a resulting effluent quality for in-mill recycling, the data can be used to simulate different effluent treatment options. These include various possibilities such as separating effluents and treating each individually, or mixing different effluents before treatment. The treatment options should be designed to allow all the treated effluent to be recycled back to the mill, thus simulating a zero effluent mill scenario. If possible, the necessary treatment plant data should be obtained from pilot plant data or from literature references if pilot work has not yet been done.

The simulation of different effluent treatment options makes it possible to compare different treatment technologies and combinations on a technical basis. The options that are found to be technically viable can be costed and compared on a techno-economical basis. This ensures that the mill is presented with the most cost effective effluent treatment and recycling option to achieve a zero effluent mill.

After selection of the most suitable treatment regime, the optimum sequence of implementation of treatment technologies has to be found. This sequence of implementation has to ensure that the mill can achieve zero effluent in a stepwise fashion, without having to re-design the whole effluent treatment system whenever an improvement in effluent quality is required. This implies that each consecutive treatment option has to produce a step change in effluent quality, until the resulting effluent is of high enough quality to be re-used in the mill, thus resulting in a zero effluent mill. The sequence of implementation, as well as the intermediate effluent qualities can be simulated using the mill model.

3. CASE STUDIES

Case Study 1 - Tugela Mill
Sappi Tugela Mill is an integrated pulp and paper mill that produces various grades of packaging paper. The mill has two pulping lines producing Kraft (pine), and Neutral-Sulphite Semi-Chemical (gum) pulp. Waste paper is also purchased and used as an additional fibre source for paper production.

The mill abstracts its process water from the Tugela River. The mill has primary effluent treatment, where effluent from the various process plants are settled in two separate clarifiers, after which they are combined into a single stream and discharged via a pipeline back into the Tugela River. The current mill layout is shown in Figure 3.1.

Figure 3.1

Figure 3.1. General layout of the Tugela Mill

In times of drought, the Tugela river flow drops significantly, meaning that the mill uses up to 70% of the total river water during these periods. It would therefore be beneficial to both the mill and the environment if the mill could reduce its water intake and effluent volume. By increasing the effluent volume, the contaminant loading in the effluent also decreases, thus further reducing the load on the river.

Simulation results
A baseline model was completed for the mill. The key components that were used are sodium, suspended solids and COD, as these are the most important components in the mill's effluent discharge permit.

Several in-mill water recycling measures were identified, and the baseline model was used to simulate the resulting effluent quality after implementation of each of these measures. Most of these recycling opportunities were found in the paper machines' white water and vacuum seal water systems. Figure 3.2 shows the projected final effluent flow and main contaminant concentrations after implementation of each recycling project.

From Figure 3.2, it is clear that the in-mill water reuse projects reduce the water consumption and effluent discharge of the mill. The final effluent volume can be reduced by 43%, and although the contaminant concentrations in the final effluent are higher, the COD, sodium and suspended solids loads in the final effluent are lowered.

Figure 3.2

Figure 3.2. Projected final effluent flow and contaminant concentration after in-mill recycling options. The changes were modelled incrementally, to ascertain which of the options would have the greatest impact on the final effluent

In order to achieve zero effluent in the mill, several treatment options were modelled. This includes biological treatment and filtering of the total effluent and recycling of this treated effluent to suitable places in the mill. However, not all of the biologically treated effluent can be recycled directly to the mill, as there are not enough suitable application points for this water in the mill. Furthermore, recycling of the effluent increases the contaminant concentration in the recycled water, thus potentially hampering product quality.

It would therefore be beneficial to treat a portion of the effluent using reverse osmosis, as this system produces a large volume of very clean water, and a small volume of brine. The high-purity water can be used as boiler feed water make-up, and the brine stream, which contains high concentrations of recoverable chemicals, can be recycled to the recovery plant. The benefits of installing a reverse osmosis plant are listed below:

  • The contaminant concentrations in the recycled effluent are lowered.
  • A significant amount of pulping chemicals is recovered by recycling the reverse osmosis brine stream to the recovery section.
  • The high purity water that is used as boiler feed water significantly reduces the load on the demineralisation plant, thus also reducing the ion exchange regeneration frequency, which itself is a source of effluent contamination.
  • The mill will achieve zero effluent status, and reap numerous benefits, as described previously.

Unfortunately, reverse osmosis is an expensive treatment option, and it would therefore not necessarily be viable to use desalination.

A possible final mill configuration is shown in Figure 3.3.

Figure 3.3

Figure 3.3. Proposed treatment scenario for Tugela mill, based on computer simulation results 

Conclusion of Tugela Mill Case Study
The use of computer simulations to attain an effluent management plan for Tugela mill has proved very successful. A wide variety of treatment and recycling scenarios were considered and modelled. The technically viable options can be compared on a cost basis, thus resulting in an optimal solution. This effluent management plan provides the mill with a roadmap to achieving a zero effluent mill, should external pressures necessitate such a step.

Case Study 2- Adamas Mill
Sappi Adamas Mill is a small paper mill that produces speciality fine paper as well as waste paper grades on two paper machines. The total production of the mill is approximately 100 tpd.

As the mill is situated in Port Elizabeth, process water is purchased from Port Elizabeth Municipality, and a fee is also paid to discharge effluent to the municipal sewer. This situation has lead to a rapid escalation in water and effluent costs.

The general layout of Adamas mill is shown in Figure 3.4. The mill uses an average of 2000 m3/d of water, of which 25% is potable water and the rest is treated sewage water. The mill treats the effluent using dissolved air flotation to remove the fibre. The fibre is recovered and a portion of the treated effluent is used in the recycled fibre paper machine. The average effluent discharge volume is approximately 1600 m3/d.

Figure 3.4. General layout of Adamas Mill

To save water and effluent costs, the water intake has to be minimised, implying that larger volumes of effluent has to be recycled back into the mill. The effluent quality is currently not of an acceptable standard to be recycled, and therefore additional treatment of the effluent will be necessary before more effluent can be recycled.

Simulation and results
In order to select the most suitable treatment option to achieve minimum municipal water usage and producing minimum effluent volume, different treatment scenarios have to be simulated and its results evaluated. For this purpose, a WinGEMS model of Adamas mill was built to produce a baseline model to be used to simulate the various effluent treatment and recycling options.

For recycling purposes, the key contaminants identified were COD, suspended solids, sodium, chloride and sulphates. Sodium and chloride is not part of the process chemistry, but enters the mill with the water and the bought in pulp. However, recycling of effluent could lead to a slight rise in the concentration of these components and therefore has to be considered. Sulphate enters the process through the addition of alum in the paper machines. This means that the concentration of sulphates could increase significantly if more effluent is recycled.

In order to recycle effluent to the paper machines, the effluent has to be of an acceptable quality to replace the incoming treated sewage water. Therefore, besides removing suspended solids, as is currently being done in the DAF, the COD of the effluent also has to be reduced. This can be achieved by means of a biological kidney, which reduces the COD of the effluent before it is returned to the mill.

Several biological treatment systems were tested using the baseline model, each with their own advantages and disadvantages. Figure 3.5 shows a typical mill layout with a biological kidney and effluent recycling. It was found that by using a biological kidney, the use of treated sewage water could be eliminated completely, without leading to excessively high TDS and COD concentrations in the recycled effluent. Table 1 shows the water use and effluent characteristics for the current mill configuration as well as a typical effluent recycling scenario.

Figure 3.5
Figure 3.5. Proposed biological kidney and effluent recycling for Adamas Mill

 

Current operation

Biological kidney

Average potable water usage (m3/day)

500

500

Average treated sewage water usage (m3/day)

1500

0

Specific water consumption (m3/ton paper)

20

5

Average effluent discharge (m3/day)

1600

100

Recycled effluent COD (mg/l)

700

300

Sodium (mg/l)

250

350

Chlorides (mg/l)

330

180

Sulphates (mg/l)

250

1300

Table 1. Water and effluent characteristics of baseline model and a simulated treatment option

As can be seen from Table 1, the water intake and the effluent discharge volume of the mill is significantly reduced. The COD level in the recycled effluent is reduced significantly, as COD is removed by the biological kidney. Although the TDS level rises quite significantly, this is not seen as a problem, as a periodical purging of effluent will keep the TDS down to an acceptable level.

Conclusion of Adamas Mill Case Study
The use of computer simulation to reduce the water intake and effluent discharge volumes at Adamas mill has proved to be very successful. Besides presenting the mill with a technically viable solution, the simulation data generated can also be used to select the most cost effective treatment option.

4. CONCLUSIONS

When searching for effluent treatment and recycling solutions for a pulp and paper mill, computer simulations provide a powerful tool to predict the effect of certain changes on the mill and its effluent. These effects are not always immediately apparent, especially when effluent is recycled back to the mill, thus affecting many aspects of the process.

Although building full mill simulations is a major task that requires time and intensive labour, the benefits of using these models as a predictive tool for effluent management are numerous.

Sappi Technology Centre has used this tool with success for several of its Southern African mills and continues to use computer simulations to provide its mills with effluent management solutions.

5. REFERENCES

1. Albert, Richard J. Restrictive Environmental Regulations Drive Mills to Operate Effluent -Free. Pulp and Paper 67(13), pp 97-99, December 1993.

2. Webb, L. Closing up the water loop without closing down the mill. Pulp and Paper International 39(6), pp 43-46, June 1997.

3. Jordan, H. Environmental regulations, pollution control and energy consumption – pulp and paper. Paper Technology 36(4), pp 3-9, May 1995.

4. Synk, Robert J. Water Reuse, Reduction Projects Save Money at Recycling Mills. Pulp and Paper 72(3), pp 93-98, March 1998.

5. Panchapakesan, B. Closure of mill whitewater systems reduces water use, conserves energy. Pulp and Paper 66(3), pp 57-60, March 1992.

6. WinGEMS software user's manual, Pacific Simulation, Moscow, Idaho, USA.

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